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FOAL HEAT BREEDING
Patrick M. McCue
DVM, PhD, Diplomate American College of Theriogenologists
board and care for barren mares during this
‘lost year’ are obvious.
Foal heat is the first estrus or heat period
that occurs after foaling. Mares usually first
come into heat by 6 to 8 days postpartum.
The average interval from foaling to first
ovulation is 10 days, although mares can
ovulate as early as 7 to 8 days or as late as
14 to 15 days postpartum and be considered
clinically normal.
Pregnancy rates are
usually higher for mares that ovulate 10 or
more days after foaling than mares that
ovulate on or before day 9 post foaling.
The Postpartum Mare
The equine uterus undergoes remarkable
changes following foaling. Placental fluid
and debris (called lochia) is expelled, the
tissue lining of the uterus regenerates and
the size of the uterus decreases. A majority
of mares have significant inflammation
present in their uterus during the first week
postpartum and bacteria can be cultured
from the uterus of many mares during their
foal heat.
Although many breeding
programs specify that mares will not be bred
unless a negative uterine culture is obtained
(i.e. the mare is ‘clean’ or free of infection),
research has indicated that the presence or
absence of bacteria is not correlated to
fertility in foal heat mares. This will not
hold true for cycles after the foal heat, where
presence of uterine bacteria is a significant
cause of reduced fertility.
The Breeding Decision
Mare owners and breeding managers must
decide as a general rule whether or not to
breed mares on the foal heat. Reviews of
breeding records indicate that mares initially
bred on their foal heat will maintain a
foaling interval of approximately 12 months.
Not every mare becomes pregnant after a
foal heat breeding, but the average interval
from foaling to pregnancy for mares initially
bred on the foal heat is about 25 days. In
contrast, waiting until the second (30-day)
heat to begin breeding will inevitably result
in a longer foaling-to-conception interval.
Owners that choose not to breed mares on
the foal heat must accept the fact that
eventually each mare will have to be left
open for a season every 4 to 6 years when
the potential foaling date becomes too late in
the season. The economic implications of
the lost foal crop and continued costs of
A majority of mares have a foal heat
ovulation and subsequently continue to
cycle at approximately 21-day intervals.
However, some mares will have a foal heat
ovulation followed by a variable period of
anestrus until the mare resumes normal
cyclic activity. A frustrating few will have
minimal to no follicular development
following foaling and will not cycle at all for
2 to 3 months. Failure to develop follicles
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A simple, common strategy to optimize post
partum conception rates in mares with
uncomplicated foalings is to have the mare
examined by ultrasound on days 7 and 9
post foaling. Mares that ovulate on or
before day 9 are not bred, but are
administered prostaglandins 5 days after
ovulation to bring them back into heat (i.e.
short-cycled). Mares that still have a large
follicle 9 days post-foaling are bred using
standard techniques. The use of ovulationinducing agents (i.e. hCG or GnRH) is
discouraged until day 9 or 10, since
ovulations early in the postpartum period
may not be as fertile.
and/or ovulate after foaling is due primarily
to the effects of season. Consequently,
mares that foal in January through March in
the Northern Hemisphere are most likely to
be affected. Failure of follicular growth in
postpartum mares has occasionally been
attributed to lactation and some mares return
to estrus soon after the foal is weaned.
Management Options
A number of strategies have been used in an
attempt to enhance fertility of mares in the
early postpartum period.
Exposure of
pregnant mares to an artificial photoperiod
during the last trimester will shorten the
length of gestation by 7 to 10 days and
decrease the risk of returning to a state of
ovarian inactivity or anestrus following
foaling. Administration of progesterone or
Regumate to postpartum mares has been
used in an attempt to delay the first
ovulation and enhance pregnancy rates, but
results have been inconsistent. It used to be
trendy to lavage the uterus of mares 1 to 2
days after foaling. However, clinical trials
concluded that routine uterine lavage did not
increase pregnancy rates in normal postfoaling mares. Oxytocin or prostaglandins
are sometimes used in post-foaling mares to
promote uterine contractions and thereby
eliminate fluid and debris and decrease the
size of the uterus in an attempt to enhance
foal heat conception rates.
In general, owners are encouraged to breed
mares during the foal heat period. However,
mares that experienced foaling or
postpartum complications (i.e. dystocia,
retained placenta, prolonged discharge, etc)
may have a decreased foal heat pregnancy
rate or an increased embryonic loss rate.
Consequently, it may be advisable to breed
such mares on the second postpartum estrus
(30 day heat) or a later heat period.
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